Use immediate add when possible in space allocator
Use immediate add when possible in space allocator

Save one instruction in allocating new space by using an immediate add if
possible to calculate the new top of heap.

BUG=

Review URL: https://chromiumcodereview.appspot.com/11091068
Patch from Anthony Berent <aberent@chromium.org>.

git-svn-id: b158db1e4b4ab85d4c9e510fdef4b1e8c614b15b@12718 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh